The present invention relates to an instrument in which two mass spectrometric units are coupled together and, more particularly, to a sweeping method for a mass spectrometer which has a front stage consisting of either superimposed fields or an electric field and a rear stage consisting of superimposed fields.
In an instrument in which two mass spectrometric units are coupled together (which will hereinafter be referred to as "MS/MS instrument"), parent ions having a specific mass are selected by the first mass spectrometric unit. The selected parent ions are caused to collide with neutral molecules in a collision chamber that is disposed in the ion path. Thus, the parent ions are dissociated, producing daughter ions which are introduced into the second mass spectrometric unit for mass analysis.
To attain a simplified structure of MS/MS instrument, an apparatus may be contemplated in which the second mass spectrometric unit consists of a single electric or magnetic field. Generally, however, when parent ions are split to produce daughter ions, the internal energy, generally known as translational energy, is released, and therefore the daughter ions have a wide range of energies. Accordingly, in cases where a single electric field is used as the second mass spectrometric unit, what can be measured is only energy in essence. In cases where a single magnetic field is employed as the second mass spectrometric unit, it is impossible to discriminate between the mass and the energy of daughter ions for measurement. In the latter case, therefore, an assumed energy is used for an ion or other means is utilized to accurately know the mass of the ion.
In view of the foregoing, the present inventor has already proposed a compact superimposed-field mass spectrometric unit as a second mass spectrometric unit (see U.K. Pat. No. 2,133,924 and U.S. patent application Ser. No. 458,756, filed Jan. 17, 1983). FIG. 1(a ) shows the structure of the proposed instrument. FIG. 1(b) is a cross-sectional view taken on line A-A' of FIG. 1(a). This instrument has an ion source 1, an electric field 2, and a magnetic field 3 which together constitute a double-focusing mass spectrometric unit of normal arrangement. A collision chamber 5 is disposed at the position of ion convergence that is created by this first mass spectrometric unit. Magnetic poles 6 and 6' are mounted between the collision chamber 5 and a collector 4 to produce a magnetic field in the direction perpendicular to the page. The magnetic poles are excited by a magnetic field power supply 7. A pair of electrodes 8 and 8' produces a toroidal electric field in the direction perpendicular to the magnetic field. The voltage applied across the electrodes is generated by an electric field power supply 9. Auxiliary electrodes 10 and 10' which are known as Matsuda plates are mounted between the magnetic poles 6 and 6' so as to surround the toroidal electric field. A compensating power supply 11 applies a compensating voltage to the auxiliary electrodes. This superimposed-field mass spectrometric unit consisting of these components 6, 6', 7, 8, 8', 9, 10, 10', 11 is arranged as the second mass spectrometric unit.
As a sweeping method for this superimposed-field mass spectrometric unit, the aforementioned British patent discloses a sweeping in which the magnetic field intensity is changed between two levels, and at each level the electric field is swept. This method permits measurement of both the mass and the energy of daughter ions, but the spread of energy of daughter ions necessarily deteriorates the resolution. A better measuring accuracy is desired. Further, it is required that the magnetic field intensity be changed between two levels.